This is the second step towards exposing OES_draw_buffers_indexed (that defines independent blend state for each draw buffer). This flag is global in all graphics APIs, however D3D11 technically puts it in the blend state.
D3D11: BlendStateKey was extended to keep existing D3D11 state caching semantics.
D3D9: a comment was added explaining why this feature was never implemented there.
Bug: angleproject:4394
Change-Id: Ie6a294eeb6fcf4c868a1f1001c4f7efd61692ccd
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2057063Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>
